A conventional p-type transistor is known in which current drive power is improved by using a SiGe crystal layer for a channel region. The conventional p-type transistor, for example, disclosed in JP-A-5-183154. When the SiGe crystal layer is used for the channel region, there is a case that another layer such as a Si crystal layer, etc., is formed between a gate insulating film and the SiGe crystal layer. One of the reasons is to suppress reliability degradation of the gate insulating film. This is because Ge in the SiGe crystal layer may diffuse into the gate insulating film when the gate insulating film is formed by applying oxidation treatment to a surface of the SiGe crystal layer, which may result the reliability degradation of the gate insulating film.
However, in a conventional transistor using a SiGe crystal layer for a channel region, since both of a SiGe crystal layer and a Si crystal layer, which is an upper layer thereof, are generally formed by using a vapor-phase epitaxial crystal growth method, the following problem may occur.
When the SiGe crystal is epitaxially grown, surfaces having different plane directions such as, e.g., a (100) plane and a (111) plane, appears. And then, when Si crystal is grown by vapor-phase epitaxy on the SiGe crystal layer, since a growth rate is different depending on a plane direction of a surface which is a base (seed) of the growth, a thickness of the crystal varies depending on portions. For example, since the growth rate of the Si crystal on the (111) plane of the SiGe crystal layer is slower than that of the Si crystal on the (100) plane, the thickness of a portion located on the (111) plane is smaller than that of a portion located on the (100) plane in the Si crystal layer.
Therefore, in the portion where the Si crystal layer is thin, a distance between the gate insulating film and the SiGe crystal layer is narrowed, which results in that Ge becomes likely to diffuse into the gate insulating film. On the other hand, if a thick Si crystal layer is formed in order to avoid such a problem, since a thickness of a portion originally having an enough thickness is further increased, a region of the SiGe crystal layer functioning as a channel region is decreased, thereby reducing an effect of improving the current drive power.